In the preparation of polymers known in the art as polyesters, one or more dicarboxylic acids and one or more glycols (or functional equivalents thereof) are reacted in the presence of a transesterification catalyst to form a relatively low molecular weight polymer, often referred to as an "oligomer", which has an average degree of polymerization of about 4 or less. Such an oligomer is conventionally prepared by an ester exchange reaction which comprises condensing a lower dialkyl ester of a dicarboxylic acid(s) with a stoichiometric excess of a glycol(s) until most of the dialkyl ester has been converted to glycol esters and oligomers.
Further heating of the oligomers under conditions facilitating removal of glycol and, optionally, in the presence of polycondensation catalysts then leads to a gradual increase in molecular weight as the oligomers polymerize by reacting with one another. At intermediate stages, such as at a molecular weight in the range of from about 2000 to about 10,000, the polymer is often termed a "prepolymer" and is in the molten state. It has been customary for those skilled in the polymer chemistry art to polymerize the prepolymer until the molecular weight of the polymer has attained a prescribed useful value such as about 40,000. Ordinarily, this value is expressed as an inherent viscosity. For example, it is known in the art that the final inherent viscosity of one useful polyester, poly(ethylene terephthalate), must be higher than about 0.5 in order for that polyester to have advantageous physical properties when extruded as a film.
It is also known that most polyesters, including poly(ethylene terephthalate), undergo thermal degradation when held above their melting temperatures for extended periods of time. Such degradation increases as the temperature is increased. Further, in the presence of oxygen, oxidation occurs which is also accelerated by higher temperatures. Although the polymerization reaction rate generally increases with temperature, the time necessary to produce a prepolymer of sufficient molecular weight for commercial use is such that undesired colored products have often resulted, seriously affecting some uses of the eventual polymer. Changing the catalyst or adding a color stabilizer has been suggested as a way for improving the polymer color. However, such attempts to control color have had limited success.
It is further known that polyesters can be prepared by a continuous melt polymerization process wherein the molecular weight build-up of the prepolymer is continued as an extension of the molten polycondensation of the prepolymer. It is also known that solid phase (i.e. powder) polymerization techniques can be utilized to obviate certain defects of the melt polymerization process, such as the unsatisfactory color of the polymer produced by high melt polymerization temperatures. Since solid phase polymerization is carried out at temperatures below the melting point of the prepolymer, thermal degradation and the resulting discoloration are greatly reduced. The main defect in the solid phase polymerization process is the relatively long reaction time necessitated by the technique.
As pointed out in U.S. Pat. No. 3,342,782 (issued Sept. 19, 1967 to Barkey), the long reaction time for the solid phase polymerization is believed to be due to the relatively low reactivity of the prepolymer produced by the melt polymerization process. In the Barkey patent, a method for improving the reactivity of the prepolymer is described whereby the prepolymer is crystallized by casting it upon an inert surface and cooling it in a carefully controlled way prior to solid phase polymerization. Although this method is effective in improving prepolymer reactivity, it has several disadvantages. Namely, it is a relatively slow technique for crystallizing the prepolymer and requires an undesirable interruption between polycondensation and solid phase polymerization. Further, it is limited to solid phase polymerization and is not adaptable to other methods of polymerization, such as melt phase polymerization.
Disclosures of typical polyester preparation methods are provided, for example, in U.S. Pat. Nos. 3,075,952 (issued Jan. 29, 1963 to Coover, Jr. et al.) and 3,390,134 (issued June 25, 1968 to Kibler). As described therein, conventional polyester preparation includes a crystallization step directly after melt phase polycondensation wherein solid particles of the prepolymer are treated with a volatile organic liquid to increase prepolymer crystallinity. Solid phase polymerization then follows. This crystallization technique, however, does nothing to improve prepolymer reactivity. Furthermore, it is similar to the process of U.S. Pat. No. 3,342,782 in that it is limited to solid phase polymerization and not adaptable to other polymerization methods.
Hence, there is still a need in the art for a method of improving polyester prepolymer reactivity, thereby shortening polymerization time and reducing the likelihood of undesirable polyester properties, e.g. discoloration. Furthermore, it is desired that such a method would provide a more reactive prepolymer which can be polymerized in any suitable polymerization process.